Updated version of the ltc2990 driver which supports all measurement
modes (current, voltage, temperature) available in the chip.

If devicetree is used, the mode must be specified with the property
"lltc,meas-mode". The format and possible values of the property are
described in the binding.

If devicetree is not used, the mode of the chip will not be configured.
Unless the chip is configured by another source, only the internal
temperature and supply voltage will be measured.
Signed-off-by: NTom Levens <tom.levens@cern.ch>
Tested-By: mike.looijmans@topic.nl
[groeck: Fixed compiler warning]
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>

This adds support for the Linear Technology LTC2990  I2C System Monitor.
The LTC2990 supports a combination of voltage, current and temperature
monitoring. This driver currently only supports reading two currents
by measuring two differential voltages across series resistors, in
addition to the Vcc supply voltage and internal temperature.

This is sufficient to support the Topic Miami SOM which uses this chip
to monitor the currents flowing into the FPGA and the CPU parts.
Signed-off-by: NMike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>